1. Field of the Invention
This invention relates generally to a graphics display in a data processing system, and more particularly to apparatus for clearing the display between successive vertical synchronization operations.
2. Description of the Prior Art
Graphics and alphanumeric text are displayed visually in many business applications. This allows the relationship between many variables of the business to be presented in pie chart or bar graph form. The graphics may also be used to display and manipulate mechanical or electronic designs.
An operator may want to view several charts in rapid succession requiring bit map memories to be cleared of the old information, after which new information is written into the bit map memories. The bit map memories, one for each primary color, store an image of the screen in typically a solid state memory.
The prior art uses a software technique for clearing each bit map memory. Since the software is not timed to the horizontal and vertical synchronization operations of the display, the image on the screen is distorted during the software clear operation. This distortion is annoying to an operator, particularly when the display is used for long periods of time. To avoid this condition, the software must first turn off the display, then clear and then turn the display back on. This same procedure may also be accomplished in hardware with cyclic operations within the hardware.